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ADP1621ARMZ-R7 Datasheet(PDF) 21 Page - Analog Devices |
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ADP1621ARMZ-R7 Datasheet(HTML) 21 Page - Analog Devices |
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21 / 32 page  ADP1621 Rev. A | Page 21 of 32 EFFICIENCY CONSIDERATIONS The efficiency, η, of a dc/dc converter is given by % 100 × = IN OUT P P η (38) where POUT is the output power, and PIN is the input power to the converter. While switching regulators are ideally lossless converters of power, the nonideal characteristics of regulator components degrade the efficiency of the regulator. The primary sources of power dissipation in the regulator include • The power dissipation in the external power MOSFET due to conduction and switching losses. SW C MOSFET P P P + = (39) + ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + × × × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = ) 1 ( 1 K R D D I DSON LOAD ⎥ ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎢ ⎣ ⎡ × + × − × + 2 ) ( 1 ) ( SW F R LOAD D OUT f t t D I V V • The power dissipation in the external current-sense resistor if lossless current sensing is not used. CS LOAD CS R D D I P × × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = 2 1 (40) • The power dissipation in the external diode. LOAD D DIODE I V P × = (41) • The power dissipation in the winding resistance of the power stage inductor. W LOAD W L R D I P × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = 2 , 1 (42) • The supply current to the ADP1621 IC, which includes the quiescent current and the gate driver charging current. The power dissipation due to gate charging loss is approximated by SW G PIN G f Q V P × × = (43) where PG is the gate charging power loss, VPIN is the voltage at the PIN pin, QG is the MOSFET total gate charge, and fSW is the converter switching frequency. Therefore, the total power dissipation in the IC itself is given by ( ) Q IN G IC I V P P × + = (44) ( ) ( ) Q IN SW G PIN I V f Q V × + × × = where PIC is the total power dissipated in the IC, IQ is the quiescent current, and VIN is the voltage at the IN pin. The secondary sources of power dissipation in the regulator include • The power dissipation in the ESR of the input and output capacitors. • Inductor core losses due to hysteresis and eddy currents. |
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